In systems which transmit data on a carrier, it is desirable to employ some sort of squelch or muting system to avoid a situation in which the carrier signal has vanished for some reason and the receiver is generating noise. The data supplied from such a receiver may be interpreted by a microprocessor, which also has several other tasks to perform, many of which apply even when data is not being received. If the microprocessor does not have to take its time attempting to interpret noise, a much more efficient system results.
For example, in an off-premises CATV system including a microprocessor, data is transmitted to the off-premises unit via frequency modulated carrier, for example at 108.2 MHz. This off-premises CATV system is more fully disclosed in U.S. Pat. No. 4,912,760 issued Mar. 27, 1990 to West, Jr., et al. and commonly assigned with the present invention. The disclosure of West, Jr., et al. is hereby incorporated by reference herein. A receiver detects the carrier and outputs baseband digital data to the microprocessor over a serial data line. If the carrier is removed, either intentionally or due to a failure, the receiver will generate a digital signal which is noise. This is similar to what is heard when an FM radio is tuned between stations, providing the radio does not automatically mute. If the digital noise is presented to the microprocessor, which is programmed to interpret data among its other chores, several undesirable things could happen. It is possible that the microprocessor will interpret the noise as a legitimate command, and it will do something it is not supposed to, such as disconnect a subscriber improperly. The probability of this circumstance may be reduced, but not eliminated, by proper coding and error checking. Another more probable outcome of the data carrier being removed, is that the microprocessor will be so busy attempting to interpret data which is in error, that it will not have time to perform other programmed chores.
This difficulty has been solved in the past in off-premises CATV system by providing an analog mute or squelch circuit as part of the analog circuits in the data receiver. Typically, a voltage proportional to the carrier signal strength is developed in the receiver which may then be monitored by a threshold detector. When the signal strength, as represented by the voltage, drops below the predefined threshold, the output from the receiver is removed or muted. This works, but has the limitation that the circuits which develop the voltage and which monitor it, are analog circuits, and subject to drift especially over the wide temperature ranges encountered in off-premises apparatus intended to be used outdoors. Further, cost is an important consideration in subscriber circuitry and the elimination of the analog squelch circuits would be beneficial for that reason.